Well, they both bother me so I'm the perfect candidate to give an opinion
Mariah, she's great and all, sure, and Christina...yeah her too. They're both wrong. They're both dissing someone. I doubt any of it's for publicity. "If I diss Mariah, people will buy me album! Teehee!" I doubt Christina said that. And I doubt Mariah's saying the same either. For example, Paris Hilton is all over hatin' on Lindsay Lohan (and I love her for it, by the way), yet I doubt she's saying "I should diss Lindsay Lohan so in so many months, people will buy my album."
People will forget about this by the time Christina's album comes out anyway.